Subterranean boring machines are used to install a pipe comprised of multiple casing sections or a similar product in the ground without excavating a trench for the pipe. Some boring machines are used to bore a generally horizontal hole and to install a plurality of pipe sections therein between a generally vertical launch shaft or pit and a similarly oriented target shaft or pit. The launch shaft or pit is excavated to a depth to permit the boring machine to be placed in alignment and on grade with the desired underground installation. Boring machines that are commonly placed in such launch pits generally include a track that is located at the bottom of the launch pit and oriented along the desired boring direction, and a carriage that rolls or otherwise travels along the track. The carriage includes a pusher mechanism that is adapted to move the carriage along the track between a start point and a terminal point, and a rotational mechanism that is adapted to rotate a tool carried by the boring machine.
In some conventional boring machines, a pilot hole is first bored along the centerline of the intended product bore. Some such machines use a small pilot head having an inclined face at its leading end. Typically, the pilot system will include a hollow casing, and the pilot head will include an illuminated target on its trailing end. A theodolite mounted at a fixed position in the launch pit apart from the boring machine is aligned so that an operator can view the position of the illuminated target on the pilot head with respect to the cross-hairs of the theodolite. The first section of the pilot casing and the pilot head are pushed into the ground in the boring direction by the pusher mechanism without rotating the pilot head. However, because the inclined face on the pilot head gives rise to a reaction force exerted by the soil through which the head moves, rotation of the head may be required to keep the head on the desired course, as indicated by the position of the target with respect to the cross-hairs of the theodolite. When the carriage reaches the terminal point of the track, the pilot head is disengaged and the carriage is withdrawn to the start point. Multiple sections of pilot casing are added, one by one, to the end of the pilot string and pushed by the pusher mechanism until the pilot head reaches the target pit. After the pilot hole is completed, a boring assembly having an outer diameter that corresponds to the outer diameter of the product casing to be installed is selected. This boring assembly, which comprises a support casing with a cutter head at the leading end and an internal material removal auger or other conveyor located behind the cutter head, is lowered into the launch pit and mounted on the carriage. The boring assembly is attached to the rear end of the pilot casing and aligned with the pilot hole. As the carriage is then pushed or driven along the track following the direction of the pilot hole, the cutter head is operated to bore a hole centered on the pilot hole, and the conveyor is operated to carry the cuttings back towards the launch pit. A plurality of spoil paddles located in a front section of the carriage sweep the cuttings out of the boring machine and into the launch pit through openings provided for that purpose. When the carriage reaches the terminal point of the track, the support casing and internal conveyor are disengaged and the carriage is retracted to the start point. The pilot head is removed from the pilot casing string in the target pit, and an additional section of support casing with an internal conveyor section is mounted to the carriage and attached to the casing and conveyor sections that were disengaged from the carriage. Then the carriage is engaged to drive the support casing further in the boring direction while operating the cutter head and the internal conveyor. Multiple sections of this cutting assembly are added, and the pilot system is removed, piece by piece, from the target pit, until the cutter head reaches the target pit. Then the support casing and conveyor sections are removed, one by one, as product pipe sections of the same outer diameter as the support casing are pushed by the carriage into the bore.
The Bohrtec subsidiary of Herrenknecht sells a machine which may be used for boring a small-diameter tunnel without first forming a pilot bore. The Bohrtec machine employs a boring assembly, carriage and track such as is employed by conventional machines that use a pilot bore, as well as a guidance system that allows an operator to view a target mounted on the back side of the cutter head through the hollow shaft of an auger conveyor. The cutter head of the Bohrtec machine is mounted to a steering module that is selected to match the diameter of the product casing to be installed. A support casing with internal auger conveyor is mounted behind the steering module to which the cutter head is attached, and the head is operated to cut the desired product bore. As the carriage pushes the cutter head in the boring direction while operating the cutter head and the internal auger, cuttings cut by the cutter head are carried by the internal auger through the support casing back to the launch pit. When the carriage reaches the terminal point of the track, the support casing and internal conveyor are disengaged and the carriage is retracted to the start point. An additional section of support casing with an internal conveyor section is mounted to the carriage and attached to the casing and conveyor sections that were disengaged from the carriage. Then the carriage is engaged to drive the steering module and support casing further in the boring direction while operating the cutter head and the internal conveyor. Multiple sections of this cutting assembly are added until the steering module reaches the target pit. Then the support casing and conveyor sections are removed, one by one, as product pipe sections of the same outer diameter are pushed by the carriage into the bore.
Yet another guided tunnel boring machine is described in U.S. Pat. No. 5,813,282. This machine includes a guided steerable head that is attached to a support casing by a plurality of hinge assemblies. The steerable head includes an inner pipe and an outer pipe. The inner pipe is fixed to the outer pipe so as to form an annular space within which the hinge assemblies are mounted. The support casing located immediately behind the steerable head also includes an inner pipe that is fixed with respect to the outer casing so as to form a continuation of the annular space around the inner pipe of the steerable head to accommodate the hinge assemblies. Steering actuators are mounted within the annular space of the steerable head and operatively connected to the steering hinge assemblies. A cutter head is mounted in front of the steerable head, and a conveyor auger is located within the inner pipe of the steerable head and the first casing, which auger is adapted to convey material cut by the cutter head towards the launch pit. Hydraulic and control lines for the steering head extend from the drive carriage along the top of the support casings to a junction box located on top of the first casing.